While the invention is subject to a wide range of applications, it is especially suited for use in amplitude modulation (AM) broadcast transmission systems. In such systems, adjacent channel interference causes two annoying types of sounds:
(1) Carrier beats, which in the United States, cause 10 kHz whistles.
(2) Splatter from overlapping sidebands.
As the receiver bandwidth and fidelity is increased, these annoying sounds become more noticeable.
Carrier beats can be reduced to a point where they are not annoying by installing notch filters which attenuate the whistles 20 db or more. Since these whistles have a narrow bandwidth, the notch may be quite narrow and, therefore, such filters do not noticeably degrade the fidelity of the desired audio signals.
On the other hand splatter from overlapping sidebands covers a wide range of audio frequencies and the use of normal filter means would seriously degrade fidelity.
The present invention relates to AM signals that intentionally have substantially more energy on one side of the carrier than the other and that have an envelope that is a reasonably linear function of the input modulating wave. While the system is most suitable for use in the transmission of monophonic signals, it can be used, with some restrictions, to transmit stereophonic programs.
A significant amount of work has been done in the field of asymmetrical sideband AM transmission, see P. P. Eckersley, "Asymmetric Sideband Broadcasting", Proc. IRE, vol. 16, pp 1041-1092, September 1938; and, L. R. Kahn, "Compatible Single-Sideband", Proc. IRE, vol. 49, pp 1503-1527, October 1961 (see other references in this paper.)
As pointed out in this 1961 publication, CSSB (Compatible Single-Sideband) waves provide the following advantages:
(1) A signal-to-noise power gain for a given audio fidelity. This may be seen by recognizing the fact that the bandwidth of a receiver for CSSB need only be 1/2 of that for AM. Therefore, white noise should be reduced by 3 db and impulse noise by some 6 db. In practice, it appears that due to economics and the requirements for reduction of adjacent channel interference, the common broadcast receiver is actually more suitable for CSSB than for double-sideband AM reception.
(2) Improved audio fidelity. Because, as mentioned above, the receivers are too narrow for AM, halving the bandwidth of the signal allows the higher frequency components to be passed, resulting in improved fidelity.
(3) Reduction in co- and adjacent-channel interference; (a) to the CSSB listener, and (b) to the station which previously experienced interference from the station now using CSSB.
(4) Provision of a means for reducing interference caused by improperly-shielded TV receivers and other devices, because, under many practical conditions, the station may choose the appropriate sideband to reduce the interference.
The present invention requires the use of both sidebands and, therefore, does not provide the spectrum saving advantage of CSSB but does offer many of the other advantages of CSSB operation.
Most AM radio receivers are relatively narrowband devices and typically 1984 car radios have audio bandwidths in the order of 2.2 kHz at the -6 db point.
In order to provide some semblence of musical fidelity for listeners to such low fidelity receivers, broadcasters have been forced to pre-emphasize the higher frequency audio components in some cases, by as much as 20 db relative to low frequency components. Accordingly, any transmission system that is designed for AM broadcast service must be able to accommodate pre-emphasis. However, the use of large amounts of pre-emphasis makes manual tuning of receivers much more difficult because even small errors in tuning can cause unpleasant, harsh sounds.
The harsh sounds that one typically hears when a receiver is detuned severely restricts the amount of detuning used. Since optimum CSSB operation requires listeners to detune their radios, this is an important problem.
It is, therefore, an object of the present invention to provide a transmission system that allows listeners to substantially detune their receivers and still hear a pleasant sound.
It is a further object of the invention to produce a modulated wave that has sufficient pre-emphasis of high frequency components so that, when the signal is received with a narrowband receiver, reasonable quality is achieved.